S. Askenazy

Composite conductors for high pulsed magnetic fields

Abstract In this paper, two kinds of reinforced conductors, with high strength and high conductivity are discussed: stainless steel clad copper wires (σuts(77 K) = 1370 MPa for 50% of stainless steel) and copper wires with 9 million continuous filaments of niobium (σuts(77 K) = 1050 MPa for 25% of niobium; resistivity ratio from 273 to 77 K: 5.43).

Non-destructive quasi-static pulsed magnetic fields at toulouse

Abstract A 1.25-MJ capacitor bank with crowbar is used to generate 43 T in a 26-mm bore in a 11-mm bore, with 1-s pulse duration. Coils are wound from ordinary copper wire and precooled by liquid nitrogen. The average life is 200 shots at maximum field. Recently, 61 T was obtained with 0.25-s duration using reinforced copper wire.

Transport properties and giant Shubnikov-de Haas oscillations in the first organic conductor with metal complex anion containing selenocyanate ligand, (ET)2 T1Hg(SeCN)4

Abstract Temperature dependence of the resistivity in various crystallographic directions and high pulsed field magnetoresistance of organic metal α-(ET) 2 TlHg(SeCN) 4 have been studied at temperatures down to 80 mK. Giant Shubnikov-de Haas oscillations, which are attributed to the two-dimensional nature of the cylindrical Fermi surface with a very small warping along the direction of the lowest conductivity have been observed. Four harmonics of the fast oscillations with fundamental frequency F 0 = 653 ± 3 T and slow frequency oscillations with F s = 38 ± 5 T have been revealed.

Pulsed magnetic fields in Toulouse: past, present and future

An overview over past and present activities and technical developments in the Toulouse pulsed magnetic field facility is given. Results obtained recently with a 60 T, 150 ms, 1.25 MJ prototype magnet tested with the new 14 MJ generator are presented.

Ultra high strength nanofilamentary conductors: the way to reach extreme properties

Abstract To enhance the intensity of non-destructive magnetic fields with long pulse duration, reinforced conductors are needed with extremely high mechanical strength and good electrical conductivity. The ideal conductors for this application should have an action integral close to that of pure copper. An elaboration process based on cold drawing and restacking has been developed at LNCMP for this purpose. The best results have been obtained with Cu/Nb nanocomposite wires with a section of 3×10 −2 xa0mm 2 composed of a copper matrix embedding 9×10 6 continuous parallel niobium whiskers with a diameter of 40xa0nm. The ultimate tensile strength is 1950xa0MPa at 77xa0K. The fundamental properties linked to the effect of nanometer size have been investigated. Nevertheless, because of their small section these conductors cannot be practically used in the winding of our magnets. Therefore, we are elaborating a new generation of optimized Cu/Nb nanostructured wires exhibiting ultra high strength in a section of 2xa0mm 2 . The latest developments are presented. Concurrently, we are developing Cu/Ta multifilamentary conductors. Since the shear modulus of tantalum is greater than that of Nb ( μ Ta ≈2 μ Nb ), the Cu/Ta UTS should be enhanced. However, drawing of Cu/Ta billets leads to the formation of a macroscopic roughness at the Cu/Ta interface and to the fracture of Ta. This phenomenon is interpreted in terms of stress-driven rearrangement (Grinfeld instabilities). We have investigated some solutions to prevent its formation.

Spin transitions in a series of FeII molecular complexes induced by a strong-pulsed magnetic field

Abstract We report on the first comparative investigations of Fe II spin-crossover complexes under high-pulsed magnetic fields near the transition temperature T c . A partial irreversible triggering of the spin conversion is evidenced when the magnetic field is applied at a temperature belonging to the heating branch of the thermal hysteresis. A strong correlation between the strength of the cooperative interactions and the kinetic character of the spin conversion process is demonstrated.

Series expansion for At(0, r) and Bz(0, r)

Abstract It is demonstrated, for a uniform current density coil [0, a, l ], that the B z (0, r ) component of the magnetic field is a r series expansion in the winding ( r ≦ a ) and a 1/( l 2 + r 2 ) one outside of the winding ( r≧a ). An equivalent type of series represents the vector potential A t (0, r ). From these formulae we conclude: the very thick coils have the highest reliability.

High magnetic field induced spin transition, H.M.F.I.S.T. effect, in [Fe0.52Ni0.48(btr)2(NCS)2]H2O

Abstract We report the effect of a high magnetic field (30xa0T) on the LS↔HS transition in [Fe 0.52 Ni 0.48 (btr) 2 (NCS) 2 ]H 2 O. We have developed a new experimental set-up which allows to determine under a high pulsed magnetic field, the proportion n HS of molecules in the high spin state ( S =2). The experiments, performed under quasi-static pulsed field, are based on the optical reflectivity as detection of the LS↔HS transition. The results on [Fe 0.52 Ni 0.48 (btr) 2 (NCS) 2 ]H 2 O are reported and show that a partial spin transition (10%) may be induced by using a pulse of 30xa0T and confirm the previous study of Fe(Phen) 2 (NCS) 2 .

A two-stage ("coilex-coilin") pulsed-magnet system with a hybrid supply

A pulsed magnet consisting of two independently energised coaxial coils has been designed, built and tested. The outer coil (coilex) is a conventional wire-wound coil energised by the power-limited supply (controlled rectification of power from the mains) available at Amsterdam. Two different inner coils (coilin) were made with soft copper and optimised internal reinforcement by fibre composites (glass fibre and Zylon impregnated with epoxy). The coilin was energised by a capacitor bank (energy limited supply). It was demonstrated that this coil system works well; peak fields well in excess of 60 T were obtained, with good prospects for further development towards higher fields.

Angular and magnetic field dependencies of the magnetoresistance of λ-(BETS)2FeCl4 in the field-restored high conductivity state

Abstract The very sharp metal-insulator transition, which occurs at 8 K at ambient pressure in λ-(BETS) 2 FeCl 4 is suppressed when applying magnetic fields >10T. This unusual phenomenon may be connected to ferromagnetic ordering of the Fe 3+ moments, which could drive some “evaporation” of the previously condensed conduction electrons, into a field-restored highly conducting state (FRHCS) in which oscillations of the magnetoresistance are reported.